This invention relates to composite drills, especially drills designed for use in automatic drill-changing equipment. More particularly, it relates to such drills of small sizes for use in drilling holes in printed circuit boards.
It is known to use drills in a wide range of sizes and types, and from a wide range of materials of varying degrees of hardness, depending on the intended use.
When very small drills have been used, persistent problems have plagued the users, especially in printed circuit board manufacturing industries where extremely hard drills are commonly used. Suitably hard, durable drills, such as those made from tungsten carbide, have proved to be extremely brittle and subject to breakage from being dropped, even short distances, on hard surfaces. This breakage, even with utmost precautions and care, seems to occur at an irreducible frequency, such that additional safeguards are counterproductively expensive (as, for example, attempting to provide soft, resilient drop surfaces on all working areas).
Breakage expenses are aggravated even more by the heavy weight and large quantity of expensive tungsten carbide (or similar materials) used in each drill. For example, with small drills having diameters less than about 1/4 inch (6.35mm), the actual working portion of a drill may constitute as little as 5%, or even less, of the total material in the drill. Heretofore, the additional 95% of expensive, heavy drill material was considered indispensable to provide a drill shank of sufficient size, hardness, durability, and dimensional stability to be compatible with precision collets. Such qualities are of special importance in the fine, precision drilling operations of the circuit board industry, especially where automated high-speed drill-changing techniques are employed.
An example of such drill-changing techniques is illustrated in U.S. Pat. No. 3,973,863, which shows highly automated drilling devices and processes in which one or more collets are each programmed to sequentially pick up a drill, use it in drilling work, such as drilling circuit boards, return the drill to its storage location, and pick up another drill to continue the sequence. For such operations, the shanks of all the drills in the set must be matched, that is, of precisely uniform size to compatibly match the collet. As shown in U.S. Pat. No. 3,973,863, this uniformity is achieved by greatly enlarging the shanks of the drills to a constant large diameter to fit the collet (regardless of the working diameter of the drill). Such enlarged shanks have been virtually essential for precision gripping and centering of the drill to avoid eccentric drill motion and to achieve the fine-tolerance drilling operations required in circuit board manufacture.
It has been found that when, e.g., drills having large shanks are composed entirely of heavy material, such as tungsten carbide, breakage upon impact is nearly inevitable if the drills are dropped even short distances. Both the heavy weight of the enlarged shanks and their incapacity to absorb shock, i.e., their lack of resilience, contribute to the breakage.
Another problem encountered when shanks are made of extremely hard materials, such as tungsten carbide, is the difficulty of properly setting collet gripping pressures. It is virtually impossible to know when a tungsten carbide shank is gripped with excessive pressure, since its surface will not dent, even when potentially damaging, excessive stresses are imposed upon it. Moreover, overtightening the collet gripping pressures can damage the collet or greatly increase its wear-rate and shorten its operable life.
Despite the long history of repeated efforts to improve drills, prior art techniques have failed to overcome these problems.